This invention relates generally to calcium carbonate for use in papermaking, and related industries, and more particularly to a calcium carbonate having acid resistant properties.
Titanium dioxide mid calcined clay have traditionally been utilized as filler materials in the preparation of neutral to weakly acidic paper in order to improve the optical properties, especially the brightness, of the resultant product. These materials, however, especially titanium dioxide, have the disadvantage of being very expensive, resulting in higher manufacturing costs and an uncompetitively priced paper product.
Calcium carbonate, particularly precipitated calcium carbonate, has been used as a filler material in the making of alkaline paper. Such usage results in a paper with enhanced optical properties, without the expense incurred in using titanium oxide fillers, resulting in a much less expensive product. Calcium carbonate, however, cannot generally be used as a filler in acidic paper because it decomposes in an acidic environment. Consequently, there has long been a need to develop a calcium carbonate composition which is acid stabilized and resistant to decomposition at low pH, so that it can be utilized as a filler material in the manufacture of acidic paper, such as groundwood paper, where the use of an alkaline filler would have a negative impact on the final paper properties.
A particular disadvantage of calcium carbonate, when used as a filler in paper, is its reaction with alum which is also commonly used in papermaking. Alum (aluminum sulfate [Al.sub.2 (SO.sub.4).sub.3 18H.sub.2 O]), produces aluminum hydroxide and sulfuric acid when added to the "wet" papermaking system. The flocculation of aluminum hydroxide is capable of collecting and retaining the filler and also part of the resin-alum system for paper sizing. During manufacture, the alum flocks in the paper web are removed, and the sulfuric acid is increasingly concentrated in the white water, in spite of the buffer action of the system. The presence of calcium carbonate in a system of pH 4 will quickly consume part of the sulfuric acid, thereby forming calcium sulfate and free Ca.sup.++ ions. The presence of these ions in the recycled white water causes a considerable reduction of the sizing effect in the papermaking pulp, and this, in turn, necessitates the use of large amounts of alum to maintain stable sizing conditions.
In the manufacture of paper, dewatering and retention were improved by adding to the stock a water-soluble aluminum chloride, aluminum hydroxychloride, viz. Al.sub.2 (OH).sub.5 Cl, or polyaluminum chloride, [Al.sub.2 (OH).sub.n Cl.sub.6-n ].sub.m. Polyaluminum chloride varies substantially according to the manufacturing procedure, and the process kinetics of degree of neutralization affect both the product produced and its reactions in the papermaking system. In neutral and alkaline papermaking systems, polyaluminum chlorides (from simple to highly complex polymers) can be used for pitch control, drainage, retention, and sizing. On the other hand, the surface charge and zeta potential of aluminum chloride are higher than those of alum from 3.5 to 8 pH range.
A variety of techniques to modify calcium carbonate to achieve acid resistance and avoid the aforementioned problems are disclosed in the art. For instance, U.S. Pat. No. 5,043,017 discloses and claims an acid-stable calcium carbonate resistant to degradation in a mildly acidic environment which comprises a mixture of a calcium-chelating agent or a conjugate base, and a weak acid such that calcium carbonate is coated by and is in equilibrium with the calcium-chelating agent or conjugate base and the weak acid. Preferred calcium carbonate compositions contain sodium hexametaphosphate and phosphoric acid. U.S. Pat. No. 5,000,791 discloses an acid-resistant coating on calcium carbonate particles consisting of a zinc compound and a solution of a silica-containing substance.